Counterion effect on the solution and thermodynamic properties of lithium perfluoroalkanoates

The isotherms of conductivity of lithium perfluorooctanoate and perfluoroundecanoate were measured and the critical micelle concentration, cmc, degree of ionization of the micelles, beta, determined in a range of temperatures above the Krafft point. The thermodynamic parameters, Gibbs free energy, Delta G(m)(0), enthalpy Delta H-m(0), and entropy Delta S-m(0) of micelle formation, were determined from a proposed thermodynamic model. Apparent molar volume and the apparent molar adiabatic compressibility for both surfactants have been calculated over a wide concentration range from density and ultrasound velocity measurements. Positive deviations of the apparent molar volume from the Debye-Huckel limiting law in dilute solutions indicate the existence of premicellar aggregation. Changes in the slope of adiabatic compressibility of lihium perfluoroundecanoate from positive to negative have been interpreted in terms of solute solvent interactions. Dynamic surface tension measurements allow us to calculated diffusion coefficients, areas and aggregation numbers. These values are discussed comparing the corresponding sodium compounds.